Mediterranean Gulls (Ichthyaetus melanocephalus) are a relatively new breeding species in western and central Europe. They are characterized by sexual monomorphism in plumage, which precludes easy sex identification in this species. To assess the degree of sexual dimorphism in body size and verify whether morphometric traits may be used to reliably sex individuals of this species, the measurements of six traits (wing, head, bill and tarsus length; bill depth at gonys; and bill depth at the base) were taken from individuals breeding at several sites located across Poland. Mediterranean Gulls were sexed molecularly by amplification of CHD-linked (chromodomain helicase DNA binding protein gene) sequences from W and Z chromosomes. The repeatability of measurements for different traits was high or very high except for bill depth at the base when measured by different observers. Males were larger than females in all traits, but distributions of these traits substantially overlapped. A discriminant function analysis selected three traits (head length, bill depth at the gonys and tarsus length) that classified the sex of individuals with 100% accuracy. It is suggested that the derived discriminant function may be successfully applied to sex Mediterranean Gulls from different breeding populations in central Europe.